There have been used various electric power supply apparatus provided with an electric power converter circuit such as an inverter, a cycloconverter or the like to convert an output from the electric power supply into an AC output of predetermined frequency. For example, as electric power source apparatuses comprising an AC generator driven by a primer such as an internal combustion engine, there have been used many electric power supplies adapted to convert an output from the AC generator into a DC voltage by rectifying the output and thereafter convert the DC voltage into an AC output having a predetermined peak value and a predetermined frequency by an inverter circuit.
Such an electric power supply apparatus comprises a rectifier to rectify the output from the AC generator, an inverter circuit including on-off controllable switch elements to convert an output from a rectifier power source portion into an AC output by a switching operation of the switch elements, a filter to remove a higher harmonic component out of the output from the inverter circuit, load connection terminals across which the output from the inverter circuit is applied through the filter, a load voltage detection circuit to detect a voltage across the load connection terminals and a controller to control the switch elements of the inverter circuit so as to obtain an voltage having a predetermined waveform across the load connection terminals.
There has been used for the inverter circuit a bridge type circuit having such a construction as a plurality of arms including upper switch elements and lower switch elements are connected in parallel to each other so as to form a bridge circuit.
The controller normally comprises a CPU (microcomputer) to control the switch elements of the inverter circuit to thereby turn them on or off at a predetermined period so as to output an AC voltage having a stepped waveform obtained by modulating a reference voltage with a pulse width modulation (PWM) system so as to simulate the waveform of the reference voltage.
What is meant by the reference voltage is such as a voltage as provides a reference waveform of an AC output that should be obtained across the load connection terminals (the output terminals of the electric power supply apparatus). This reference voltage is one having the waveform similar to that of the desired AC output.
The controller, for example, may comprise PWM (pulse width modulation) signal generation means to generate a PWM signal which in turn determines a duty value of the respective switch elements of the inverter circuit for obtaining from the inverter circuit the AC voltage of waveform provided by modulating the reference voltage with the PWM system by on-off controlling the switch elements of the inverter circuit with the predetermined duty value, PWM signal correction means to correct the PWM signal so as to have no deflection between a momentary value of the voltage detected by the load voltage detection circuit and a momentary value of the reference voltage, switch element drive means to apply a drive signal to the respective switch elements so as to on-off control the respective switch elements of the inverter circuit with the duty value determined by the PWM signal and overcurrent protection means to protect the switch elements by stopping the drive signal from being supplied to the inverter circuit when it is detected that the load current exceeds a set value.
The PWM signal normally includes a rectangular waveform signal having a repetition of a first state such as a high level state, for example and a second state such as a zero level state at a predetermined period. The PWM signal has the first and second states for the off-state period of the switch elements of the inverter circuit and for the on-state period thereof, respectively.
In the specification, the period of the PWM signal is called "PWM period" and the frequency of the PWM signal is called "PWM frequency". A ratio of time where the PWM signal has the first state relative to one period of the PWM signal is called the duty value of the PWM signal while a ratio of time occupied by the on-state time of the switch elements in the respective PWM period is called the duty value of the switch elements.
Furthermore, a time when the respective switch elements of the inverter circuit get the on-state or when the PWM period begins is called a switch time.
In case that the controller comprises the CPU, the respective PWM period can be detected by counting pulses generated at a predetermined period by a PWM period counting counter. The time when the respective PWM periods begin becomes the switch time when the switch elements get the on-state.
The duty value of the PWM signal generated at the respective switch times can be read out from a duty arithmetical operation map which is stored in a ROM to provide a relation of a series of switch times, an output voltage of the rectifier and the duty values of the PWM signal at the respective switch times or the duty values of the switch elements. Otherwise, it can be obtained by being arithmetically operated by using an arithmetical operation expression.
The CPU of the controller sets the on-time of the switch elements for the PWM signal generation timer on the duty value obtained by being read out from the map with an internal interruption of the operation which may be made at every PWM period "t" and provides a potential of the first state such as the high level state to an output port of the PWM signal while the on-time set by the timer continues to be counted whereby the PWM signal is generated.
The switch element drive means applies the drive signal (the signal for providing the on-state to the switch elements) to the corresponding switch element during the period of the first state of the PWM signal.
Supposed that the frequency (PWM frequency) of the PWM signal generated at the period "t" is referred to as "fp" and that the frequency of the output waveform is referred to as "fo" (period T), the internal interruptions will be made at times of n=fp/fo during the period T of one cycle of the output waveform.
FIG. 3 shows a relation between an internal interruption time (that is the switch time of the switch element) and the duty value of the PWM signal in case that the reference voltage is of sine waveform. In this figure, "a" designates a reference voltage waveform, "t" designates a PWM period, "VA" designates a peak value of the reference voltage, "Vav" designates an average value of the reference voltage and "T" designates a period or cycle of the reference voltage (a period or cycle of the desired AC voltage).
The duty value of the PWM signal varies for every time "t" as the momentary value of the reference voltage changes and thus the AC voltage of stepped waveform which is obtained by dividing the waveform of the reference voltage of one cycle into "n" waveform portions and which corresponds to the waveform obtained by modulating the reference voltage with the PWM system is output from the inverter circuit. As the AC voltage of the stepped waveform passes through the filter, the high harmonic component is removed out thereof and therefore, the output voltage of smooth sine waveform is obtained across the load connection terminals.
As the frequency of the PWM signal gets higher, the number of times of the interruption during one cycle of the AC voltage will increase. This enables the waveform of the reference voltage to be more finely simulated, which causes the waveform of the output voltage to be smoother. However, since the frequency of the PWM signal is required to be decided in consideration of the delay time such as the turn-on time or the turn-off time of the switch elements after the controller generates the signal till the switch elements are actually operated or the performance of the CPU such as the internal operation time thereof, the frequency of the PWM signal cannot get higher to an unlimited extent. Normally, the frequency of the PWM signal is set at about 10 KHz and constants of an inductance "L" (of coils) and a capacitance "C" (of capacitors) for the filter are decided in accordance with the frequency of the PWM signal.
In the electric power supply apparatus for generating the AC voltage with the inverter circuit used, there occurs a delay after the switch elements of the inverter circuits are turned on or off with the predetermined duty value until a voltage appears across the load connection terminals due to a delay of the on-operation and off-operation of the switch elements for the inverter circuit or a delay in the filter, which causes a phase difference to occur between the reference voltage and the voltage across the load connection terminals. FIGS. 4A and 4B show the relation of the phase between the waveform "a" of the reference voltage and the waveform "b" of the voltage across the load connection terminals. FIG. 4B shows a portion "B" of FIG. 4A in an enlarged scale. It will be noted from FIGS. 4A and 4B that the voltage across the load connection terminals is generated with a delay time of .DELTA.t relative to the reference voltage.
In the electric power supply apparatus using the inverter, whenever every internal interruption is made, the waveform of the voltage across the load connection terminals is converted by an A/D converter into a digital signal and read into the CPU. The momentary value of the thus read voltage across the load connection terminals makes an arithmetical feedback operation so that it is coincident with the momentary value of the reference voltage whereby the duty value of the PWM signal is corrected. In this case, since the CPU reads in the voltage across the load connection terminals with the time delay .DELTA.t relative to the reference voltage, the arithmetical feedback operation is made in consideration of the time delay .DELTA.t relative to the reference voltage so that the voltage across the load connection terminals is not distorted.
In the prior electric power supply apparatus, since the voltage across the load connection terminals is just controlled by turning on or off the switch elements of the inverter circuit with the timer provided in the CPU, the phase of the voltage across the load connection terminals cannot be coincident with that of an external voltage which will be applied across the load connection terminals due to the operation of the electric power supply apparatus in parallel to a separate electric power supply or due to disturbance.
Thus, when the external voltage is applied across the load connection terminals due to the operation of the electric power supply apparatus in parallel with the separate external electric power supply or due to the disturbance, there sometimes occurs a phase difference between the output voltage and the external application voltage, which provides a short circuit between the inverter circuit and the external electric power supply. This causes an overcurrent to flow through the switch elements of the inverter circuit, and at the worst breaks the switch elements.